docs: clarify withdraw preconditions

[babymakh2026-maker]

Summary

• Document the difference between modeled ResultAsync validation outcomes and thrown SDK invariant errors.
• Clarify in the package README, client withdraw action docs, and useWithdraw docs that withdraw assumes the sender already has a supply position for the selected market and reserve.
• Point client and React users to userSupplies / useUserSupplies before submitting a withdrawal.
• Update the touched examples to use the current sender request field.

Why

Issue #150 surfaced that consumers can mistake thrown invariant failures, such as attempting to withdraw without an existing supply position, for execution-plan validation errors. The maintainer guidance is that these are violated preconditions, while modeled outcomes such as InsufficientBalanceError remain part of the normal execution flow.

This PR makes that contract explicit where integrators are likely to read it, so they know which checks to perform before calling withdraw and avoid depending on invariant message text.

Refs #150.

Validation

• pnpm lint
• pnpm build